Lithographic printing plate precursors which are used mainly in the filed of small-scale commercial printing include (1) a direct drawing type printing plate precursor having a hydrophilic image-receiving layer provided on a water-resistant support, (2) a printing plate precursor having provided on a water-resistant support a lipophilic image-receiving layer comprising zinc oxide, which is converted into a printing plate by undergoing direct drawing image formation and then desensitizing treatment with a desensitizing solution to render the non-image area hydrophilic, (3) a printing plate precursor of an electrophotographic light-sensitive material having provided on a water-resistant support a photoconductive layer comprising photoconductive zinc oxide, which is converted into a printing plate by undergoing image formation and then desensitizing treatment with a desensitizing solution to render the non-image area hydrophilic, and (4) a printing plate precursor of a silver-halide photographic material having a silver halide emulsion layer provided on a water-resistant support.
With the development of office appliances and the expansion of office automation in recent years, it has been desired in the field of printing to adopt an offset printing system wherein a lithographic printing plate is directly prepared from the printing plate precursor of direct drawing type (the foregoing (1)) utilizing various image forming means, e.g., an electrophotographic printer, a heat-sensitive transfer printer or an ink jet printer without undergoing any other special treatment for conversion into the printing plate.
Further, another method for direct preparation of a printing plate wherein an electrophotographic printer is utilized has been proposed. More specifically, in an electronic editorial system wherein input, correction, editing, layout and pagination are performed by a continuous computer operation and the resulting image information is instantly transmitted into terminal plotters in distant places via a high-speed communication network or a communications satellite, an electrophotographic printer adaptable to digital signal input is used as the terminal plotter, and a printing plate is prepared directly from the output of the printer.
Recently, an ink jet recording method rapidly spreads because of its ability of low noise and high-speed printing.
With respect to the ink jet recording method, various ink jet systems, e.g., a so-called electric field controlling system in which ink is ejected utilizing electrostatic attraction, a so-called drop-on-demand system (pressure pulse system) in which ink is ejected utilizing an oscillation pressure of a piezoelectric element, and a so-called bubble (thermal) jet system in which ink is ejected utilizing a pressure developed by bubbles produced and grown by means of high thermal energy have been proposed, and these systems can provide images of high accuracy.
A conventional lithographic printing plate precursor of direct drawing type comprises a support, such as paper, having on one surface side an image-receiving layer which is a surface layer provided via an interlayer and on the other surface side a back layer. The interlayer and the backlayer are each composed of a water-soluble resin, such as PVA or starch, a water-dispersible resin, such as a synthetic resin emulsion, and a pigment. The image-receiving layer comprises an inorganic pigment, a water-soluble resin and a water resisting agent.
Examples of inorganic pigment used include kaolin, clay, talc, calcium carbonate, silica, titanium oxide, zinc oxide, barium sulfate and alumina.
Examples of water-soluble resin used include polyvinyl alcohol (PVA), modified PVA such as carboxylated PVA, starch and derivatives thereof, cellulose derivatives such as carboxymethyl cellulose and hydroxyethyl cellulose, casein, gelatin, polyvinyl pyrrolidone, vinyl acetate-crotonic acid copolymer, and styrene-maleic acid copolymer.
Examples of water resisting agent used include glyoxal, initial condensates of aminoplasts such as melamine-formaldehyde resin and urea-formaldehyde resin, modified polyamide resins such as methylolated polyamide resin, polyamide-polyamine-epichlorohydrin adduct, polyamide epichlorohydrin resin, and modified polyamide-polyimide resin.
In addition to the above described ingredients, it is known that a cross-linking catalyst such as ammonium chloride or a silane coupling agent can also be used.
However, for improving printing durability of a printing plate obtained by a conventional manner as described above, if the hydrophobicity of the printing plate is enhanced by adding a large amount of the water resisting agent or by using a hydrophobic resin, printing stains due to the decrease in hydrophilicity (affinity of the plate for water) occur although the press life is improved. On the contrary, the enhancement of hydrophilicity results in lowering of the water resistance to cause deterioration of press life.
In particular, when the printing plate is used under a temperature condition of 30.degree. C. or more, it has a defect that the surface layer thereof is dissolved in dampening water used for offset printing to result in deterioration of press life and occurrence of printing stains. Moreover, since images are drawn directly on an image-receiving layer of a printing plate precursor with oil-based ink in the case of direct drawing type lithography, poor adhesion of the oil-based ink to the image-receiving layer causes separation of the oil-based ink from the image area during printing, thereby deteriorating the press life even if the occurrence of printing stains in the non-image area is prevented because of sufficient hydrophilicity. This problem does not yet come to a satisfactory solution.
With respect to the ink used for forming images on a conventional lithographic printing plate precursor of direct drawing type in accordance with an ink jet recording system, water-based ink which uses water as the main solvent and oil-based ink which uses an organic solvent as the main solvent are ordinarily employed.
However, the water-based ink has drawbacks of blurring the images on the printing plate precursor and causing a decrease of drawing speed due to slow drying. In order to overcome such drawbacks, a method of utilizing oil-based ink containing a nonaqueous solvent as a dispersion medium is disclosed in JP-A-54-117203 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
This method, however, is still insufficient, because image blurs are actually observed on the plate obtained, and further blurs are generated in printed matter upon printing. In addition, the number of printed matter obtained with the printing plate is on the order of several hundreds at the most, which is much lower than the desired level. Moreover, the ink has a problem of being apt to clog a nozzle for ejecting so fine ink droplets as to form images of high resolution.
In the ink jet recording system, the ink is usually passed through a filter and then ejected from a nozzle. Thus, this system tends to cause ejection troubles depending on various factors such that the nozzle or filter is liable to be clogged, the ink-fluidity changes with the lapse of time, and so on.
Such ink ejection troubles are caused by not only a water-based ink compositions but also an oil-based ink composition. For preventing the ink ejection troubles, various proposals have been made. For instance, for preventing the ink ejection troubles in the case of using an oil-based ink composition in the ink jet recording system of electric field controlling type, it is proposed that the viscosity and specific resistance of the ink composition are controlled as described in JP-A-49-50935. It is also proposed that the dielectric constant and specific resistance of a solvent used for the ink composition are controlled as described in JP-A-53-29808.
Further, as attempts to prevent clogging of the nozzle due to ordinary oil-based ink for a printer in the ink jet recording system, methods of improving dispersion stability of pigment particles (as described, e.g., in JP-A-4-25573, JP-A-5-25413, and JP-A-5-65443), methods of incorporating specific compounds into ink compositions (as described, e.g., in JP-A-3-79677, JP-A-3-64377, JP-A-4-202386, JP-A-7-109431) have been proposed.
However, even if any of the ink compositions according to those methods is used for image formation on a printing plate precursor, the images formed suffer from insufficiency of strength during printing, so the resulting lithographic printing plate cannot have a satisfactory press life.
On the other hand, in the case of adopting a platemaking method wherein images are formed on a printing plate precursor having a zinc oxide-containing image-receiving layer by an appropriate method and then the non-image area is treated with a desensitizing solution, the image on the printing plate and printed matter have good quality and a great number of printed matter having good quality can be provided. However, this method is accompanied with the complication in wet processing. Specifically, it is essential for the method to use the desensitizing solution in the course of platemaking and dampening water containing the same desensitizing component as the desensitizing solution at the time of printing. In addition, it occurs, though depends on printing ink used, that the component in the dampening water used has interaction with some component in the printing ink to tend to cause stains in the printed matter. Thus, this method has a problem of being unsuitable for color printing with a wide variety of printing inks.
In the field of digital adaptable electrophotographic printer, remarkable technical improvements have been made lately. For instance, reproduction of high resolution image have been achieved by an electrophotographic printer using fine dry toner having a particle size of 6 to 8 .mu.m, and reproduction of highly accurate images with a high reproducibility have been achieved by an electrophotographic printer using liquid toner.
In a system of image formation on a printing plate precursor of direct drawing type by image transfer using, e.g., a laser printer of such a system as described above, therefore, it is required that both prevention of background stains in the non-image area after transfer and good image reproducibility in the image area be achieved to provide printed matter having clear images without background stains, in great numbers. Further, it is desired that printed matter having a wide variety of color images be easily obtained.
Furthermore, it is requested to simply carry out the desensitizing treatment for the non-image area in the preparation of printing plate.